Laser Raman/fluorescent device for analyzing airborne particles

ABSTRACT

A laser Raman/fluorescent electro-optical device is described for counting, sizing, weighing and assaying airborne particles, whereby the physical state of the atmosphere may be monitored.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.483,786 filed June 27, 1974, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to optical devices for monitoring the physicaland chemical state of the atmosphere, and more particularly toimprovements therein.

Optical scattering techniques provide a particularly attractive methodfor monitoring the physcial and chemical state of the atmosphere. Suchmethods have a natural simplicity and lead to reliable and economicalhardware configurations. Rayleigh lidar techniques, for example, providean excellent means for detecting cloud and smog layers, locatingtemperature inversions and aerosol layers, and determining cloudstructure through rain and fog cover. Raman lidar instruments, on theother hand, offer a tool for selectively detecting and measuringatmospheric pollutants, such as carbon monoxide, sulphur dioxide, andoxides of nitrogen. Finally, Rayleigh light-scattering particle countersprovide a convenient technique for detecting and sizing individualaerosol particles.

RAYLEIGH-SCATTERING TECHNIQUES

Rayleigh particle counters measure a wide range of particle sizes (0.3microns and larger), operate at high counting rates (on the order of1000 particles per second), and offer good sizing accuracy (± 30 percentfor tungsten sources and ± 3 percent for laser sources). These devicesoperate by passing aerosol particles through a collimated laser beam ora focused tungsten source. Light scattered by an aerosol particle iscollected by a mirror or lens and then relayed to the photo-cathode of aphoto-multiplier tube. The photomultiplier current pulses are thenamplified, shaped, counted, and histogrammed by appropriate electroniccircuitry. The number of counts registered per second indicates theparticle concentration, and the histogram provides a size-frequencydistribution for the detected particles.

The chief drawbacks of the conventional light-scattering particlecounter are its inability to measure particle mass and its inability todetermine particle chemistry. To ascertain the mass and chemistry of anaerosol particle, one must measure the differential--rather than thetotal--Rayleigh-scattering cross section. Additionally, the differentialcross section must be measured at several critical wavelengths, that is,wavelengths where the suspected chemical components have strongabsorption bands. Using these measurements and their assigned errors,one can attempt to separate the scattering effects caused by the shapeof the particle from the scattering effects caused by its complexrefractive index. By studying the behavior of the complex refractiveindex as a function of wavelength, one can estimate the particle massand speculate on its chemical composition.

The above procedure--though workable in theory--gives rise to certaintechnical difficulties in practice. In particular, to attain reasonablecounting rates, one must abandon the angular and wavelength scanningtechniques used in commercial nephelometers. Instead, a hardwarearrangement that simultaneously measures the scatteredlight intensity atseveral polar angles and several wavelengths must be realized. Thenumber of angles and number of wavelengths at which samples must betaken depends on the physical and chemical makeup of the particle.(Oddly shaped particles with many chemical components requirewell-sampled, high-precision cross-section measurements.) Owing to thepractical difficulty of building such an instrument, little hope is heldfor weighing and assaying aerosol particles with Rayleigh-scatteredlight.

OBJECTS AND SUMMARY OF THE INVENTION

An object of this invention is to provide apparatus which can weigh andassay airborne particulate matter.

Another object of this invention is the provision of novel apparatus foruse in air-pollution detection and measurement.

Still another object of this invention is to provide apparatus wherebythe mass and chemistry of aerosol particles may be determined.

The method used by this invention for determining the mass and chemistryof an aerosol particle is to examine the Raman spectrum of the scatteredlight. Raman scattering differs from Rayleigh scattering in that themolecules of the particle shift the frequency of the incident light inthe Raman case but leave it unaltered in the Rayleigh case. Thefrequency shift results from small nonlinearities in the polarizabilityof the scattering molecule. The nonlinearity causes mixing of theincident-light oscillations and vibrational-rotational motion of themolecule. Accordingly, the scattered light contains upper (anti-Stokes)and lower (Stokes) sidebands about the exciting frequency. Typically,one uses visible or ultraviolet light to illuminate the target moleculesand looks for sidebands displaced above and below the exciting line byinfrared frequencies.

The infrared frequencies modulated onto the exciting line are highlyspecific to the scattering molecule and independent of the excitingfrequency. These frequencies arise when the molecule makes a quantumtransition between one vibrational or rotational state and another owingto the influence of a perturbing external electric field. In practice,the Stokes line appears more intense than the anti-Stokes line, since,according to the Boltzmann distribution, the excited molecular stateshave much smaller populations than the ground state.

Hexane, for example, has a strong C--H stretching line located 2880 cm⁻¹below the exciting frequency, whereas polystyrene has a strong C═Cstretching vinyl line at 1632 cm⁻¹ below the exciting frequency.Accordingly, one might attempt the detection of individual hexanedroplets or polystyrene spheres by illuminating these particles withcollimated laser light and then examining the spectrum of theRaman-scattered light. Similarly, to weigh such particles, one couldmeasure the intensity of the Raman-scattered light and then use thetabulated scattering cross section to determine the mass of theparticle.

Resonance scattering offers dramatic increases in the counter's diametersensitivity. With the resonance technique, one illuminates the targetparticle with laser radiation near one of the atomic transitionfrequencies of, say, a carbon or nitrogen atom. Owing to the strongabsorption of incident photons, the Raman scattering cross sectionincreases as much as six orders of magnitude, so that one can sizeparticles as much as two orders of magnitude smaller than withnonresonance techniques. Accordingly, resonance scattering might allowone to weigh and assay particles on the order of 0.1 microns. (One canuse either a tuned-dye or Zeeman-split laser to secure coincidencebetween the laser-emission and atomic-transition frequencies.)

Fluorescent-scattered light may also be used to determine the mass andchemistry of an airborne particle. As in Raman scattering, the moleculesof the target particle shift the frequency of the incident light,thereby giving rise to radiation characterizing the chemistry of theparticle. The fluorescent process begins when the incident light--withor without the aid of thermal process--boosts the illuminated moleculesto a higher energy state. After a time the energized molecules beginreturning to their original ground state. During return the moleculescascade from energy level to energy level by emitting thermal andvisible photons. Typically, one uses visible or ultraviolet light toilluminate the target molecules and looks for fluorescent radiation atoptical and infrared frequencies.

Unlike the Raman effect, the fluorescent spectrum emitted by anilluminating particle depends both on the chemistry of the targetparticle and the frequency of the exciting light. Accordingly, researchspectroscopists commonly publish two curves when reporting fluorescentspectra. One, the excitation spectrum, specifies the fluorescentintensity at a particular wavelength as a function of the excitingfrequency; the other, the emission spectrum, gives the fluorescentintensity as a function of the emission wavelength for a particularexciting frequency. Accordingly, the fluorescent technique features anextra degree of freedom when compared with Raman spectroscopy. That is,the fluorescent spectroscopist can select both an exciting frequency andemission wavelength to optimize his sensitivity to a given chemicalcomponent of the target particle.

Fluorescent spectroscopy also features extremely high sensitivities,often 10 - 1000 times more sensitive than comparableabsorption-spectroscopy techniques. The fluorescent detection limits onthe various aromatic compounds show particularly high sensitivities. Forexample, the detection limits for most polycyclic aromatic hydrocarbonsand the aldehydic and ketonic derivatives range around 10 nanograms. Theidentification limits of the polycyclic and heterocyclic hydrocarbonsrange around 1 nanogram; those of the polynuclear aromatic amines rangearound 10 - 1000 nanograms. Although impressive, even bettersensitivities obtain when one uses a laser as an illuminating source anda nondispersive, low f-number spectrometer to collect the scatteredlight.

In accordance with this invention a laser beam is used to sequentiallyilluminate airborne particles brought to the analyzing apparatus by anappropriate aerosol-handling system. The light scattered by theilluminated particles contains both unshifted (Rayleigh) and shifted(Raman and fluorescent) components. The Rayleigh component indicates thephysical presence and nominal size of the scattering particle; the Ramanand fluorescent components measure, in addition, the mass and chemistryof the target particle. The detection and sizing operations are carriedout by collecting the Rayleigh-scattered light and measuring itsintensity with a photo-multiplier tube. Similarly, the weighing andassaying operations are accomplished by measuring, respectively, theintensity and frequency of the Raman and/or fluorescent-scattered light.(The frequencies observed in the Raman-scattered andfluorescent-scattered light correspond to the chemical components of theparticle; the intensity at each frequency is proportional to the mass ofthat chemical component present in the particle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an embodiment of the invention.

FIG. 2 is an enlarged sectional schematic view of a scattering chamberand aerosol injection system suitable for use with this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, there may be seen a block diagram of anembodiment of this invention. A continuous wave laser system 10, directsa laser beam at the scattering region 12, (the details of which areshown in FIG. 22), where injected aerosol particles scatter Rayleigh,Raman, and fluorescent radiation toward a collecting lens 14. A beammonitor circuit 16 is used to monitor the intensity of the laser beam,generating a signal representative thereof and feeding it back to the CWlaser system 10 for the purpose of maintaining the beam intensityconstant.

The collecting lens 14 collimates the light and directs it toward a 45°dichroic filter 18. The dichroic filter separates the Rayleigh componentfrom the Raman and fluorescent components by reflecting the Rayleighcomponents toward a Rayleigh photomultiplier 20. Raman and fluorescentcomponents pass through the dichroic filter and then through a narrowband interference filter 22, onto the Raman-fluorescent photomultiplier24.

The output of the Rayleigh photomultiplier is applied to a signalamplifier 26, whose output is applied to a pulse shaper 28. The shapedpulses are then applied to an area integrator 30. The area integratoroutput is applied to a pulse-height analyzer 32, and also to a pulsecounter 34. To determine the particle concentration, the pulse countermultiplies the number of pulses counted per second times a geometricalconstant. The geometrical constant is determined by the geometry of theaerosol stream, the shape of the laser beam and the cubic feet per unittime of aerosol passing through the chamber, and is the reciprocal ofthe volume of aerosol passed through the laser beam each second. Thecount is converted to an analog current by a digital-to-analog converter35. The result is displayed by a damped ammeter 36 called a "ParticleConcentrationMeter."

Narrow band filter 22 is selected to pass the frequencies of interestwhen it is desired to measure Raman illumination and is selected to passthe frequencies of interest when it is desired to measure fluorescentillumination. Thus, two narrow band filters 22 are required, one forRaman illumination measurement and the other for fluorescentillumination measurement. The equipment that follows the narrow bandfilter is the same for both measurements, but measures either Raman orfluorescent radiation, as determined by the bandwidth of filter 22.

The output of the photomultiplier 24, which responds to either Raman orfluorescent radiation, as passed by filter 22, is applied to a signalamplifier 36. The output of the signal amplifier is applied to a pulseshaper 38, and the pulse shaper output is applied to an area integrator40. The area integrator pulse integrates each pulse and its output isapplied to a pulse counter circuit which counts the pulses andmultiplies them by a geometric constant. The resulting count isconverted to an analog current by a digital-to-analog converter 42, theoutput of which is applied to a damped ammeter called a "ParticleConcentration Meter" 43.

The output of the integrator circuit 40 is also applied to the pulseheight-analyzer 32. The pulse height-analyzer output is applied to ateletypewriter 44.

The area integrator 30, effectively measures the area of each electronicpulse which in turn is proportional to the cross-sectional area of thescattering particle. The pulse-height analyzer 32, histograms theintegrated pulses thereby producing a Rayleigh size-frequencydistribution curve. The pulse-height analyzer is also used to histogramthe integrated Raman/fluorescent pulses, thereby producing amass-frequency distribution curve. When operating in the automated mode,the pulse-height analyzer periodically outputs its distribution curvesto the teletypewriter for permanent data recording.

Another purpose of the interference filter 22, to which the Raman andfluorescent scattered light is applied, is to select a given Ramanfluorescent line from the several present, thereby making the instrumentspecific to a given chemical compound. Thedichroic-filter/interference-filter combination provides a Rayleighrejection ratio better than 10⁸ and a Raman/fluorescent transmission ofapproximately 50%. The pulse-counter circuit records the number of lightpulses received per unit time and displays the result in particles of aspecific chemistry per unit of volume with a current meter.

FIG. 2 is an oversized view in cross section of a scattering chamber andaerosol injection system which may be used with the embodiment of theinvention. Collimating baffles 50, 52, by way of example, limit thewidth of the laser beam, here represented by dashed lines 54. The beampasses through the center cavity 60 of the scattering chamber structure12, strikes a mirror 56 in the beam absorber, which reflects the beam tothe photodetector 58 in the beam monitoring circuit 16.

The scattering chamber 12 comprises walls, defining at the centerthereof the cavity 60, which comprises the scattering chamber. Aconstant flow of air is pumped into a tube 62, coaxially within which isa second tube 64, which necks down to a capillary portion 66. Thecapillary portion terminates just above the region through which thelaser beam passes. The walls of the block 12 have openings therein topermit the passage of the laser beam therethrough so that it will passthrough the scattering chamber and then strikes the mirror 56. Anothertube 68, which is diametrically opposite the tube 62 and the capillarysection 68, provides an exit for air and aerosol.

A suitable volume of the aerosol to be analyzed, is applied by suitablepumping means and is introduced into the airstream by means of thecapillary section 66. The airstream thus is used to form a protectivejacket or sheath around the aerosol stream. The boundary layer so formedalso ensures laminar flow, suppresses turbulence, and maintains theaerosol stream width. Additionally, the air sheath prevents aerosolparticles from landing on optical components thereby increasing thestray light level. By way of example, the ratio of the air to theinjected aerosol may be made on the order of 100:1. The injectioncapillary may be made on the order of 0.2mm in diameter and 10mm inlength. The width (0.2mm) of the aerosol stream is made considerablysmaller than the width (approximately 1.5mm) of the laser beam.Accordingly, the intensity profile traversed by an aerosol particle isnearly independent of the point of injection.

As described above, light scattered by an aerosol particle passes viathe lens and dichroic filter to the photocathodes of the photomultipliertubes. The duration of each photomultiplier pulse is determined by thetime required for a particle to cross the laser beam. Roughly speaking,the pulse length τ equals the distance 2r_(O), (r_(O) = beam radius)travelled divided by the aerosol flow velocity, V_(O). Within broadlimits, the traverse time remains independent of the size and shape ofthe particle.

The shape of the recorded pulse corresponds to the intensity profiletraversed by the aerosol particle. For a radially symmetric,Gaussian-shaped laser beam, the intensity profile also looks Gaussian.Accordingly, particle size can be estimated by examining either the peakpulse amplitude or the total pulse area. The pulse-area technique leadsto better signal-to-noise ratios (SNRs) since high frequency noise isaveraged away when integrating for the pulse area. (Higher SNR provides,of course, more sensitivity to small particles.)

The electronic circuits shown and described in connection with FIG. 1are standard, well-known, and commercially available circuits. The areaintegrator circuit for example may constitute a differential amplifierhaving a capactive-feedback configuration wherein each pulse isintegrated. Following the integration the capacitor is discharged and asignal having a fixed width but with an amplitude equal to the voltageon the capacitor is obtained.

The particle concentration display circuits count the pulses generatedby particles with diameters above a specific but variable size. Thecount is converted into an analog current which may be considered asproportional to the concentration of particulate matter. This is passedthrough a damped ammeter whereby the concentration of particulate mattermay be continuously displayed. With knowledge of the aerosol flow rateinto the scattering chamber and of the particle count per unit of time,the number of particles per unit volume can be determined.

The pulse-height analyzer sorts signals received from the areaintegrator according to amplitude and then displays the signals as ahistogram on an oscilloscope screen. By way of example, but not to beconsidered as a limitation upon the invention, a suitable pulse analyzeris a commercial device made by Northern Scientific Co. (Model 602) whichallows automated operation of the laser Raman/fluorescent particlecounter. In particular, this device contains an internal clock thatperiodically directs the analyzer to output its accumulated histogram tothe teletypewriter 44. After printing the histogram, the analyzer beginsforming a new histogram.

The nominal or equivalent diameter of the various particles isdetermined by measuring the height or area of the electronic pulsescaused by the aerosol particles. Roughly speaking, the pulse height andarea increase as the square of the particle's root-mean-square diameter.

The mass of the various particles is determined by measuring theintensity of the Raman-shifted spectral lines. The intensity of theseline varies directly with the number of molecules in a particle,assuming that the particle is chemically pure. The histogram providesthe intensity measurement from which mass is determined.

As previously pointed out, upon proper excitation of a particle it willfluoresce at a particular wavelength or wavelengths depending on theparticle composition and the exciting frequency. Curves are availablewhich indicate the fluorescent intensity as a function of the emissionwavelength for a particular exciting frequency. Thus, by the selectionof an appropriate central frequency and a moderate spectral bandwidth,one can detect the presence of a particular spectral line indicative ofthe presence of a particular chemical.

There has accordingly been described and shown herein a novel and usefuloptical device for counting, sizing, weighing and assaying airborneparticles.

I claim:
 1. Apparatus for determining the physical and chemical state ofaerosol particles comprisinglaser means for establishing a light beam,means for passing a stream of aerosol particles through said light beamwhereby each said particle generates Raman and fluorescent radiation inaddition to Rayleigh radiation dichroic filter means for separating saidRaman and fluorescent radiations from said Rayleigh radiation, bandpassfilter means in the path of said Raman and fluorescent radiations forpassing a selected portion of either the Raman or fluorescent radiationtherethrough having frequencies in accordance with the chemicalcomposition of the particles whose physical and chemical states it isdesired to be determined. photomultiplier means for generating pulsesignals responsive to the radiation from each particle that has passedthrough said bandpass filter means, means to which the output of saidphotomultiplier means is applied for producing a display of particles inthe stream passing through said laser beam sorted in accordance with thesize of the masses, means to which the output of said photomultipliermeans is also applied to producing a display indicative of the massconcentration of particles in the stream passing through said laserbeam, Rayleigh photomultiplier means for generating a pulse signalresponsive to the Rayleigh radiation from each particle, and meansresponsive to a plurality of said pulse signals for providing a displayrepresentative of the particle size distribution for a given volume ofsaid aerosol particle stream.
 2. Apparatus for determining the physicaland chemical state of aerosol particles in a stream comprisinglasermeans for establishing a light beam, means for passing a stream ofaerosol particles through said light beam whereby each said particlegenerates Raman and fluorescent radiation in addition to Rayleighradiation, dichroic filter means for separating said Raman andfluorescent radiation from said Rayleigh radiation, interference filtermeans in the path of said Raman and fluorescent radiation for enablingRaman or fluorescent radiation having a predetermined frequency to passthrough, said predetermined frequency being a function of the chemicalcomposition of the particle, photomultiplier means for generating apulse signal responsive to the Raman or fluorescent radiation from eachparticle that has passed through said interference filter means, meansfor integrating each pulse signal generated by said photomultipliermeans to provide pulses each having an amplitude representative of themass of a particle, means to which the output of said means forintegrating is applied for sorting said pulses in accordance withamplitude and displaying said amplitude sorted pulses as a histogram,means to which the output of said means for integrating is applied forcounting pulses having an amplitude in excess of a predetermined leveloccurring over a predetermined interval, means for coverting the numberof pulses counted by said means for counting pulses having an amplitudein excess of a predetermined level to an analog signal representativethereof, means for displaying said analog signal indicative of the massconcentration of said particle, Rayleigh photomultiplier means forgenerating a pulse signal responsive to the Rayleigh radiation from eachparticle, and means to which the output of said Rayleigh photomultipliermeans is applied for providing a display representative of the particlesize distribution for a given volume of said aerosol particle stream. 3.The method of determining physical and chemical properties of aerosolparticles comprisingestablishing a high intensity light beam of apredetermined diameter, passing a stream of said aerosol particlesacross said light beam to produce Rayleigh, Raman and fluorescentradiations for each particle, separating said Raman and fluorescentradiations from said Rayleigh radiation, choosing a selected portion ofeither the Raman or fluorescent radiation from the remainder of saidRaman and fluorescent radiations which has been separated from theRayleigh radiation, said selected portion having frequencies indicativeof the presence of predetermined chemicals in said particles, generatinga first pulse signal from the portion of of the selected radiation ofeach particle, producing a first display from said generated first pulsesignals which is representative of the mass concentration of aerosolparticles in the stream passing through said light beam, producing asecond display from said first generated pulse signals which indicatesaerosol particles in accordance with the size of their masses,generating a second signal from the Rayleigh radiation of each particle;and producing a third display from said second generated pulse signalsresponsive to Rayleigh radiations, representative of particle sizedistribution for a given aerosol particle stream volume.
 4. The methodas recited in claim 3 wherein each of the steps of generating a firstpulse signal and a second pulse signal, includesintegrating each of saidfirst and second pulse signals to produce integrated first and secondpulse signals, respectively.
 5. The method as recited in claim 4 whereinsaid step of producing the first display includescounting the number ofsaid first integrated pulse signals over a predetermined amplitudeoccurring within a predetermined interval to obtain a pulse count,converting said pulse count to an analog signal, and displaying saidanalog signal
 6. The method as recited in claim 4 wherein said step ofproducing the second display representative of the size of the masses ofsaid particles includesmeans for sorting said first integrated pulsesignals in accordance with their sizes.
 7. A method of determiningproperties of aerosol particles, the steps comprising:establishing ahigh intensity light beam of a predetermined diameter; passing saidlight beam through a stream consisting of a stream of aerosol particlessurrounded by a stream of air to produce Rayleigh, Raman and fluorescentradiation from the aerosol particles; separating the Rayleigh radiationfrom the Raman and fluorescent radiations; selecting a portion of eitherthe Raman or the fluorescent radiation; utilizing the Rayleigh radiationto provide data related to the sizes of the aerosol particles; andutilizing the selected radiation portion to provide data related to themasses of the aerosol particles.